The present invention relates to a layered heating article formed of a material exhibiting a positive temperature coefficient of resistance.
The present invention relates generally to heating elements, and more particularly to a self-regulating heating article which utilizes a material exhibiting positive temperature coefficient (PTC) of resistance.
The distinguishing characteristic of PC materials is that on reaching a certain temperature (switching temperature), a sharp rise in resistance occurs and the heating article utilizing such materials switches off.
There exists a need for flexible strip heaters with high power output densities and/or higher operating temperatures. One approach to electrical heating appliances involves forming a PTC material into a two-dimensional sheet and attaching to it a pair of strip electrodes, one at each end of the PTC sheet. The actual wattage delivered by such prior art heater is far less than that which would be expected because the heat is produced in a very thin band between the strip electrodes. Such a phenomenon, which is termed "hotline" by Horsma et al in U.S. Pat. No. 4,177,376, results in an inadequate heating performance and renders the heating appliance useless where high wattage outputs and/or temperatures above 100.degree. C. are desired. The aforesaid United States patent avoids this hotline problem by interposing a constant wattage (CW) layer between a PTC layer and an electrode.
It is still desired that the thermal resistance between electrodes be as small as possible for more efficient operation. Improvement in the manufacture of PTC heating appliances is further desired for cost reduction.